In power amplifiers of mechanical or mechanico-hydraulic type as employed in clamping devices of the above-described construction (see German Laid-open Patent Specification No. 23 46 771) the secondary element of the power amplifier performs a relatively short power stroke. This can be attributed to the power transmission ratio of the power amplifier. This short power stroke, which is only about 1 mm, is too small to be able to insert workpieces comfortably between, and remove them from, the clamping means. For this reason and so that the clamping means can also be used for workpieces of different sizes, an adjusting arrangement is provided in the known clamping device. This adjusting arrangement consists of an adjusting spindle screwable in a nut. The nut itself is adjustable stepwise in the stationary housing, while a continuous or infinitely variable fine adjustment can be carried out with the adjusting spindle. In order to shift the clamping member in the first place over a larger initial or starting distance (starting stroke) until it bears against the workpiece, it is necessary in the known clamping devices to turn the adjusting spindle. Apart from the fact that manual rotation of the adjusting spindle, in particular in mass production, is troublesome, there are not inconsiderable difficulties if it is desired to drive the adjusting spindle by motor in order to increase the starting stroke, i.e. the width of opening of the clamping device, in this way.
A clamping device with a power amplifier is also known (German Patent Specification No. 23 64 912) wherein the power amplifier is arranged in a housing movable in the stationary part of the clamping device. The powr amplifier is equipped with a primary plunger which can be pushed in at one end of the amplifier under the action of a push rod and with a secondary plunger emerging in the process in the same direction at the other end. Moreover, the clamping device has a mechanism having a retaining action on abutment of the clamping member against the workpiece and locking the housing containing the power amplifier with respect to the stationary part. If the movable housing with the power amplifier is to be moved until abutment occurs against the workpiece to be clamped, the locking mechanism must come into the retaining position precisely in that position in which the clamping member comes into abutment against the workpiece if the action of the power amplifier is to take full effect. Now if parts with a coarse tolerance, such as, for example, castings, are to be clamped, the supporting shoulder required for locking must be brought into such a position that the clamping device also still locks faultlessly in the case of the workpiece of largest size. In the case of parts of the smallest size, however, only part of the power stroke of the power amplifier then takes effect and, consequently, also only part of the clamping power thereof. This known clamping device is therefore only suitable for clamping mass-produced workpieces which show very small differences in size, such as, for example, pre-worked workpieces.
The problem underlying the invention is to provide a pneumatically driven clamping device, in particular a machine vice, of the kind mentioned at the beginning which has a relatively large opening stroke and ensures faultless clamping of mass-produced workpieces with larger differences in size with a clamping power of the same magnitude in each case.